1. Field of the Invention
The invention relates to a method for preparing HgCdTe substrate surface layers in order to reduce the surface damage that is typical of HgCdTe surfaces regardless of the growth or surface preparation techniques.
2. Description of the Prior Art
Hg.sub.1-x Cd.sub.x Te is a compound semiconductor with potential to meet the requirements of many infrared detector applications. This is mainly due to it's tunable bandgap, which determines its spectral operating range and it's long wavelength cutoff. The bandgap of HgCdTe can be continuously varied for 0. to 1.6 eV by changing the relative concentrations of HgTe and CdTe, i.e., changing the x-value function to utilize the tunability of the energy gap. However, the surface of HgCdTe alloys provides sites for generation-recombination which, in turn, severly limit the performance of these alloys for many infrared detector applications. Improperly treated HgCdTe device surfaces can be a source of leakage currents, excessive l/f noise, and device instability both chemically and electrically with time. Consequently, it is necessary to prepare a high quality surface on HgCdTe device materials in order to reduce the surface leakage currents as well as to maintain the chemical and electrical stability.
The continued development of HgCdTe ternary alloys is becoming increasingly important as a source of sensor materials for advanced infrared focal plane applications. The advancement of this technology requires improved H.sub.1-x Cd.sub.x Te material preparation methods, i.e., surface processing and growth techniques, as well as surface passivations and junction formation processes. Surface quality remains a critical limitation in Hg.sub.1-x Cd.sub.x Te detector fabrication. The depth of the damaged surface layer from surface processing of HgCdTe wafers is a function of the processing techniques, that is whether one does straight chemical etching, chemical-mechanical polishing or contactless polishing of the material surface. Many of the previous processes left a damaged surface layer which extended to a depth of 1000 .ANG. or more into the Hg.sub.1-x Cd.sub.x Te material. One approach for reducing the level of surface damage is to anodize the Hg.sub.1-x Cd.sub.x Te surface using a wet chemical approach and then to consume the damaged surface region by an appropriate etching technique. However, the depth of the anodization must be very accurately controlled however to prevent shifting of the damage, underlying near surface bulk Hg.sub.1-x Cd.sub.x Te material, further into the bulk Hg.sub.1-x Cd.sub.x Te. The long term wet etching of Hg.sub.1-x Cd.sub.x Te surfaces also changes the surface stoichiometry, which impacts device passivation and surface leakage phenomena. Therefore, this approach to improved surface quality for Hg.sub.1-x Cd.sub.x Te alloys is far from satisfactory.
The present method of preparing an improved Hg.sub.1-x Cd.sub.x Te surface layer is disclosed herein below.